I recently red a post by the GM Oracle (http://www.gmoracle.com/?p=619). In his post, he comments how mass combat is unrealistic a fantasy setting. His point is "proven" by an example of a single wizard with a wand of fireball. In many ways, he is right. A single fireball (or 50 in a wand) will decimate a unit of simple commoner.
In a med-fan setting, adventurers rarely tend to stick around one place for very long. After a battle or two, they drift off seeking treasure and - as their name implies - adventure.
Instead of thinking in terms of a human army (with clerics and wizards), first, let's think of an orc horde. The vast majority of its numbers is composed of disorganized individually dangerous creatures.
Why do I disagree?
Communication is key to any battle plan. When running small-scale action, it is easy to use magical means to coordinate. However, when the numbers grow, such cohesion and means of communications become confusing at best. Imagine 10, 20 or more people each talking, or relaying information at the same time.
Communication is not just knowing how engaged troops fare, but also where the reserve are, when and where to commit them, knowing about the enemy's movements. If a general cannot see where his troops are, it is impossible to provide support when and where needed.
Good examples of this includes Napoleon's victory at Austerlitz where he maneuvered his army while his Russian opponent was hampered by the fog, and assumed Napoleon would not risk battle because of it.
2- Limited Magical Means
It is false or simplistic to assume that only one side has access to magical means. Both sides would be likely to have destructive, scrying and dispelling magics. The rules in D&D and Pathfinder make such means of roughly equal level. Requiring a minimum of 5th level for the most common large-scale destructive magics (fireball), thus making it uncommon.
Next, people with limited, valuable resources (such as spell casting) rarely seek to expend those frivolously (like PCs do). Wasting fireballs on a mob of commoners is not something I would generally wish to do.
3- Target Priority
Same as modern artillery firing blindly at an enemy is not ideal. WWI showed us, massive barrage achieve only limited success unless backed up with an infantry assault (read about the creeping barrage tactics and the fall offensive of 1918).
YES, as a wizard, I can just throw my fireballs at whatever moves, leaving me with much less to fight against real tougher opponent (remember that wizard on the other side I mentioned earlier?).
4- Modern Military Tactics
GM Oracle's post mentions that fantasy warfare would be very similar to modern warfare. Good quality modern armies (such as the US) have one thing in common: they are a volunteer, professional military force. This means they are highly-trained, well-equipped and well-disciplined. Very similar to adventurers.
When gathering his military forces, a fantasy lord would call upon a few groups of well-trained warriors (such adventurers, knight-retainers and religious orders), but most of his forces would consist of local militia and commoner with little to no training, equipment and discipline.Thus the "mob" or "big block" formation makes the most sense.
Those elite groups would use tactics that would be more in line with modern warfare, yes, absolutely. But the poor little farm boy who leaves his farm to serve his lord with his club or pitchfork...
Knowing the enemy and his resources is crucial, even in fantasy warfare. The presence of high level spell casters in a city is something that is known. The presence of someone who can drop a mountain from the sky rarely goes unnoticed. People talk. Casters scry. Spies listen and observe.
Anyone quickly notices adventurers coming to town and selling for 1,000s of gp's worth of gear and loot. Merchants and people talk. They do so without malice, its just common sense. Since those merchants would have an influx of magic items for sale, they would spread tales designed to attract customers.
This forces an attacking commander to do something about those very dangerous individuals: assassins, distractions, bribery or even plain old magic. Nothing prevents an attacker from using many of those methods. PCs and high-level adventurers form wild cards that must be dealt with.
It also assumes that those around the PCs and other important NPCs haven't fallen prey to the means mentioned above. While it is often true that a PC could take command of the army, morale is likely to drop cohesion may not be the same. Why take out the impossible god-like adventurer when you can take down his few aides.
PCs can rarely be everywhere at once and an attacker would try to take advantage of that. Feints, counter-feints and all kinds of stratagems would still be used. Opponents would know that. Add to that illusion magic that would enhance those stratagems.
6- Castles and fortifications
Castles and fortifications protect little from a flying wizard/dragon, it does protect one from bandit and more "conventional" attackers. Adding machicolations and other defenses would help. At least delay such an attacker.
Elements that would change would include more elaborate underground structures to protect from airborne attackers and those underground structures would need additional defenses to keep enemies away. Do I hear "dungeon"?
Castles also make a great statement to the local populace that you are here to stay and defend against invaders (as they did in England after the Normand Conquest).
Plus... if we're very honest... They are really cool to think, design and research...
What we agree on
I think the biggest point we both agree on is that fantasy warfare cannot be modeled exclusively on medieval warfare, even though it takes a lot from it. Camps and bases would be more spread-out to avoid decimation by fireballs.
Personally, I see med-fan battlefields as blocks of lowly troops in the centers with elite forces and splinter "modern" groups fighting on the wings, seeking to gain an advantage. The result is a hybrid of both methods. Thus the basic elements of medieval warfare such as supplies, communications, logistics, organizations and the stratagems are all there, but use of magic makes such stratagems more complex, and in the end interesting.
In the end, I guess that it is simplistic to merely assume that common medieval tactics should be discarded simply because of the presence of magic. As usual, the answer is more complex.